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Contact-type Surface Roughness/Profile Measuring Instruments

Contact-type Surface Roughness

In contact-type instruments, the stylus top makes direct contact with the surface of a sample. The detector tip is equipped with a stylus, which traces the surface of the sample. The vertical motion of the stylus is electrically detected. The electrical signals go through an amplification and digital conversion process to be recorded.

To precisely measure delicate shapes and roughness with a contact-type surface roughness tester, the radius of the stylus tip must be as small as possible with small contact pressure.

Styluses are made of sapphire or diamond and their tip radius is usually about 10 μm or smaller. A conical shape with a ballpoint tip is considered ideal for a stylus.

Tip radius: r tip = 2 μm, 5 μm, 10 μm
* Cone taper angle: 60º, 90º

* Unless otherwise specified, cones on ideal measuring instruments have a 60º taper.

Advantages and Disadvantages

Contact-type surface roughness testers provide reliable measurement, because they directly touch the sample. However, direct contact also causes disadvantages as outlined above. More detailed explanations for some of these points are presented on the following pages.

Advantages Disadvantages
  • - Clear wave profile
  • - Capable of long distance measurement
  • - Stylus wear
  • - Measuring pressure can cause scratches on the sample surface
  • - Inability to measure viscous samples
  • - Measurement limited by radius of stylus tip
  • - Measurement takes time
  • - Difficulties in positioning and identification of subtle measuring points
  • - Requires sample cutting and processing for tracing by the detector

Stylus wear

The stylus must be polished, because it will wear down over time. The mode of wear varies, making the stylus flat or rounded depending on the material and shape of the measurement target object. Different stylus shapes will naturally generate different wave profiles.

One method for determining stylus wear is to use a commercially available wear inspection test piece. Wear is determined by comparing the data profile (groove width) of the test piece before and after the wear of the stylus.

Markings on the sample from measuring pressure

As explained before, styluses are made of sapphire or diamond, such hard materials can cause scratches on the surface of the test object. Especially when repeating parallel adjustments, it is easy for the stylus to scratch the sample during rapid feed.

Grooves narrower than the radius of the stylus tip cannot be measured

The tip of the stylus is spherical. The stylus tip cannot trace the shape properly if the width of the groove (scratch, etc.) is narrower than the radius of the stylus tip.

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